1. Field of the Invention
The present invention relates to a RAKE receiver device used in a mobile communications system employing spectrum spreading technology.
2. Description of the Related Art
In communications method employing spectrum spreading technology, modulated and spread data (i.e., a signal) is sent from a sending end, and the data received on a receiving end is de-spread and demodulated, whereby sending and receiving of data is performed. In the communications method employing such spectrum spreading technology, the frequency spectrum of the data is spread into a wide range so that excellent anti-interference property and communications privacy are ensured, thus having an advantage that multiple users can share a channel in the same frequency band. Therefore, in recent years, this communications method has been widely utilized in a mobile communications system such as a portable telephone or the other thing.
An example of the communications method utilizing this spectrum spreading technology includes CDMA (Code Division Multiple Access).
Incidentally, in mobile communications, wireless communication using radio waves is performed between a base station and a terminal such as a portable telephone or the other thing. In this case, the radio waves may arrive at the receiving end by propagating linearly or by being reflected by a construction such as a building or the other thing for example. Therefore, the receiving end receives multiple signals which have been transmitted through a plurality of paths (multiple paths). Since these radio waves have time lags (phase differences) caused by the differences in the distances they have traveled, a phenomenon called fading occurs such that they strengthen each other when they are in phase with each other, and when they are out of phase from each other they weaken each other.
By contrast, in the mobile communications system that utilizes the spectrum spreading technology, there is used a method called a RAKE receiver system, in which a plurality of receive data transmitted through multiple paths are synthesized to thereby improve quality of communication. A RAKE receiver device which employs the RAKE receiver system has a plurality of fingers corresponding to the number of associated paths, and each of the fingers demodulates receive data from each associated path. The phase differences of the received and demodulated data are corrected, and the data which phase differences have been corrected are synthesized.
For example, a receiver device, a receiver method and a terminal device of a portable telephone system disclosed in JP 10-209919 A, and a CDMA system communications apparatus disclosed in JP 11-331124 A are known as conventional techniques employing the RAKE receiver system.
Each of the RAKE receiver devices disclosed in these publications has one memory provided for each finger, for storing the receive data. The receive data from each path and demodulated by the finger for each path is held in each memory associated with that finger. After the receive data from the chronologically last path has been stored in the memory, identical receive data are read out from all memories, and the identical receive data from all the paths are added together and synthesized.
Thus, in the conventional RAKE receiver device, a massive amount of memory is used for each finger. Therefore, there was a problem that the scale of the circuitry increased, and, as a result, power consumption was enormous as well.